Plastics, as a kind of widely used material, have increasingly drawn people's attention. As described in the article written by Souheng Wu of DuPont Co. in US, published in POLYMER INTERNATIONAL VOL. 29, No. 3, p229–247 (1992), plastics may be classified into pseudoductile plastics and brittle plastics due to the different characteristics and nature of macromolecular chains. Plastics with a chain entanglement density (Ve) of less than about 0.15 mmol/ml and a characteristic ratio (C∞) of greater than about 7.5 belong to a brittle plastics, where the external impact energy is dissipated mainly by forming crazes from the matrix. On the other hand, plastics with a chain entanglement density (Ve) of greater than about 0.15 mmol/ml and a characteristic ratio (C∞) of less than about 7.5 belong to pseudoductile plastics, where the external impact energy is dissipated mainly by generating the shear yield from the matrix. The toughness of either pseudoductile plastics or brittle plastics can be further improved by blending with rubbers.
Many scientists have made extensive research on the theory and method for toughening plastics. In 1980's, Souheng Wu proposed the percolation model for plastics toughening, which showed that the brittle-tough transition in the plastics takes place when the distance τ among the dispersed rubber particles is less than a certain critical distance τC. Since the relationship between the distance τ among particles of the rubber phase and the diameter of the particles of the rubber phase (d) meets the following formula: d=τ[k(π/Φr)1/3−1]−1, the brittle-tough transition takes place when the diameter of the rubber particles (d) is less than the critical diameter dC. In other words, the smaller the size of the dispersed rubber particle is, the more easily the brittle-tough transition occurs to the plastics to be toughened. In the prior art of using rubbers to toughen plastics, the rubber serves as a toughener and blends with the plastics to obtain the toughened plastics. For example, U.S. Pat. No. 4,517,319 disclosed that DuPont Co. in U.S. selected polyurethane elastomers to toughen polyoxymethylene; EP 120711 and EP 121407 disclosed the Hoechst Co. in German selected diene graft polymer elastomers to toughen polyoxymethylene; EP 117664 disclosed the ASAHI KASEI KABUSHIKI KAISHA in Japan selected styrene block copolymer elastomers to toughen polyoxymethylene; and FR8519421, FR8803877, FR9512701 and FR9609148 of ATOCHEM Co. in France, JP127503/97 of MITSUI CHEMICALS INC. in Japan, JP190634/97 and JP190635/97 of KISHIMOTO SANGYO CO. LTD. in Japan, disclosed the techniques of using rubbers to toughen plastics, such as using maleic anhydride-grafted ethylene-propylene rubber to toughen polyamide. However, the above-mentioned patents have the drawbacks as follows: (1) at the present technology level, it is difficult to control the particle distribution of the dispersed rubber phase within a narrow range and the size smaller than 200 nm. A larger amount of rubbers is necessary for the brittle-tough transition, thereby leading to the decrease of stiffness of the toughened plastics; (2) the particle size of the rubber phase is unstable, that is to say, the particle size of the rubber phase always varies with the change of the processing parameters such as the shear rate during the processing; (3) the particle size of the rubber phase is far from uniform; (4) the rubber content cannot exceed 40 percent, otherwise it will lead to the occurrence of the “sea-sea” morphological structure, even the reversal of phase, which leads to the inferior properties of the toughened plastics.
In addition, since toughness and stiffness are two important mechanical properties of the plastics, how to largely improve the toughness of the plastics while keeping the desired stiffness, i.e. to obtain the materials with the balance of stiffness and toughness, is always the aim desired. At present, the method that can effectively improve toughness of the plastics is using elastomer materials to toughen the plastics, for example using EPR or EPDM to toughen PP, using acrylate rubber to toughen polyester, and the like. However, using elastomers as tougheners will simultaneously decrease the stiffness of plastics, such as flexural strength and flexural modulus and the like. Until now, there is no report on the improvement of both toughness and stiffness by only using elastomers such as rubbers.
In order to improve toughness of the plastics while keeping their stiffness, the process of blending rubbers with rigid inorganic fillers (such as mica, talc etc.) are generally used to modify the plastics. In other words, toughness of the plastics is improved by the elastic rubber phase while the decrease of stiffness caused by the addition of the rubber phase is compensated by the inorganic fillers added. However, when inorganic fillers are used for the purpose of reinforcement, the amount of the fillers used is generally relatively large (above 20 weight parts on the basis of 100 weight parts of plastics), which will impose various adverse influences on the toughened plastics, such as increasing the density of plastics, making the processing properties of the toughened plastics inferior, and the like.
In addition, inorganic rigid particles may also be used to toughen some plastics (the plastics with certain toughness) while keeping stiffness of the plastics from decreasing, i.e. the so-called rigid particles-toughening method (see Dongming Li and Zongneng Qi, “The fracture of CaCO3 reinforced polypropylene composite”, Polymer Materials Science & Engineering, 1991, No. 2, p18–25). However, as far as the rigid particles-toughening method concerned, its toughening effects is very limited, and the method is not applied at an industrial scale and is still under exploratory development.
Inorganic nano-particles may also be used for the purpose of toughening while keeping the stiffness. For example, ACTA POLYMERIC SINICA, No. 1, p99–104 (2000)(Chinese) discloses the use of nano-SiO2 for polypropylene toughening which has both toughening and reinforcing effects on PP at room temperature when the content of SiO2 is from 1.5 to 5 percent. However, during the industrial processing, the use of inorganic nano-particles for toughening plastics still causes some problems such as the relatively poor dispersion in the resin matrix, thereby influencing the final toughening effect.